Controls on moss evaporation in a boreal black spruce forest

Abstract

[1] Mosses are an important component of the boreal forest, but little is known about their contribution to ecosystem carbon, water, and energy exchange. We studied the role of mosses in boreal forest evapotranspiration by conducting two experiments in a black spruce forest in Fairbanks, Alaska. Moss evaporation was measured using lysimeters filled with Hylocomium splendens or Sphagnum capillifolium. Microclimate and moisture content were varied by placing the lysimeters in different habitats (dense forest, open forest, bog), and by manipulating the water supply (no water, natural rainfall, water added). Moss evaporation rates between 1 June and 8 September averaged 0.3, 0.9, and 1.5 mm day À1 in the dense forest (Hylocomium), open forest (Hylocomium and Sphagnum), and bog (Sphagnum) respectively. Assuming a total forest evapotranspiration rate of 2 mm day À1 , this study shows that moss evaporation contributes considerably to boreal black spruce forest evapotranspiration. Moss evaporation rates depended strongly on the openness of the forest and to a lesser degree on the density of the vascular plant canopy and on moss species. The strong influence of habitat suggests that microclimate is the primary factor determining moss evaporation rates. Hylocomium evaporation reacted strongly to experimental water additions, indicating that precipitation frequency is an important factor in addition to microclimate for this species. The large moss evaporation rates in this study suggest a potential cooling effect of mosses, of Sphagnum in particular.

Effects of Fine-Scale Topography on CO2 Flux Components of Alaskan Coastal Plain Tundra: Response to Contrasting Growing Seasons

“…Furthermore, standing water reduces the surface albedo, increasing water and soil surface temperature and potentially thaw depth. Conversely, low soil moisture reduces soil temperatures and thaw because of the low thermal conductivity of dry organic soils, particularly beneath Sphagnum moss (Brooker and van der Wal, 2003;Heijmans et al, 2004). In a warm, dry year such as 2005, active layer thickness and subsurface soil temperatures can be lower than those during a cool, wet year such as 2006.…”

“…However, in the medium-term, indirect effects of climate warming on the water table via increased evapotranspiration (Rott and Obleitner, 1992) and deeper active layer depths have the potential to be as great or greater than direct effects on the CO 2 balance. Although increased evapotranspiration as a result of warming could have a slight cooling effect on the soil surface (Heijmans et al, 2004), potentially reducing ER in the short-term, in the long-term, warming could lower the water table and increase the depth to permafrost, strongly increasing oxidation of the organic matter and limiting water access (Johansson et al, 2006;Schuur et al, 2008).…”

“…Although climate, soil type, and topography are among the primary drivers of permafrost presence and active layer thickness (Gomersall and Hinkel, 2001;Johansson et al, 2006), plant community composition, particularly the presence of bryophytes such as mosses, also plays an important role in regulating soil temperatures and depth of thaw (Beringer et al, 2001;Heijmans et al, 2004). Mosses such as Sphagnum influence the energy transfer between atmosphere and soil as a result of the cooling effect on the soil produced by high evaporation rates and the insulating effect of the thick organic layers (Heijmans et al, 2004;Gornall et al, 2007). As a result, mosses can potentially reduce depth of thaw, ER, and plant productivity (Brooker and van der Wal, 2003) by reducing soil temperature.…”

Long‐term effects of climate change on vegetation and carbon dynamics in peat bogs

“…Evapotranspiration (ET; in mm.month -1 ) is calculated for each species group as they contribute differently to total evapotranspiration. When the cover of vascular plants increases, the total evapotranspiration may be reduced, because the presence of vascular plants reduces wind speed and solar radiation at the moss surface, thereby reducing evaporation rates (Heijmans et al , 2004. However, in dry conditions vascular plants may continue transpiration while moss evaporation is strongly reduced.…”

The Impact of Moss Species and Biomass on the Growth of Pinus sylvestris Tree Seedlings at Different Precipitation Frequencies

“…In addition to potential negative effects (i.e., competition), mosses may have several positive effects on tree seedlings that vary with depth and shoot density. As such, a high moss biomass will retain water better [11,21,33], which may reduce seedling desiccation in dry periods. The moist environment within mosses has also been shown to support N 2 -fixation and N mineralization processes [14,15] that could potentially increase nutrient availability for seedlings.…”